The present invention is related to the field of magnetron sputtering. More specifically, the present invention is directed to methods and systems for providing magnetic fields within a magnetron sputtering device to achieve high target utilization.
Magnetron sputtering is a technique for coating objects that generates a stream of coating material by sputtering a target through the use of a plasma discharge. Sputtering is a process in which material is dislodged from the surface of a material by collision with high-energy particles. In magnetron sputtering devices, high-energy particles in the form of plasma ions are directed towards the target under the action of an imposed magnetic field. Sputtering is controllable through the proper application of plasma parameters, such as pressure, power, and gas, and a magnetic field, which may also be controllable. In vacuum, the sputtered materials travel from the magnetron toward one or more workpieces and adhere to the workpiece surface. Through the judicious choice of plasma gases, magnetic design and physical layout, a wide variety of materials, including metals, semiconductors and refractory materials, can be sputtered to desired specifications. Magnetron sputtering has thus found acceptance in a variety of applications including semiconductor processing, optical coatings, food packaging, magnetic recording, and protective wear coatings.
Commonly used magnetron sputtering devices include a power supply for depositing energy into a gas to strike and maintain a plasma, magnetic elements for controlling the motion of ions, targets for generating coating material through sputtering by the plasma, and provisions for mounting or holding one or more workpieces for coating. Sputtering is accomplished with a wide variety of devices having differing electrical, magnetic, and mechanical configurations. The configurations include: various types of electrodes, one of which may be the target; sources of DC or AC electromagnetic fields or radio frequency energy to produce the plasma; and permanent magnets, electromagnets or some combination thereof to direct the ions. In addition, the vacuum chamber is connected to a vacuum pump and a gas supply for controlling the environment within the chamber. Target materials used with DC or mid-frequency AC sputtering are chosen from conductive elements or alloys which form conductive materials, such as metals, metal oxides and ceramics, and typically include, but are not limited to, silver, tin, zinc, titanium, chromium, or indium. Non-conductive materials may be sputtered using RF sputtering methods.
In practice, a plasma is struck within the vacuum chamber, and magnetic fields are used to accelerate ions in a plasma onto a target, thus enhancing sputtering from the target. In addition to sputtering the target, ion bombardment heats the target and other components. The performance of electrodes, magnetic elements and targets may be improved when the various components are cooled. This cooling helps to control temperature dependent material properties that might alter or degrade the magnetic field and also increases the stability and lifetime of components. When properly maintained, the electrodes and magnetic elements generally have long lifetimes, on the order of a decade or more. The targets must be replaced when new materials are to be sputtered, or when sputtering has reduced the thickness of the target to depletion or unacceptable levels. Thus the magnetic elements have relatively low maintenance requirements as compared to the target, which must be replaced at regular intervals during normal use.
The location and strength of magnetic fields, especially adjacent to the target surfaces, have great practical importance in magnetron sputtering devices. It is well known in the art that the interaction between the change in shape of the sputtered surface and the magnetic field over the target surface results in an acceleration of sputtering at locations where sputtering has begun. Thus it is common for targets to erode rapidly at certain locations, leaving other locations relatively uneroded. The faster the sputtering, the quicker the thickness of the target is eroded. As a result, a target with increased thickness is sometimes used to prolong the target lifetime. The increased thickness may increase the amount of material available for sputtering, but can adversely affect the total percentage of target material consumed during sputtering. The fraction of target material consumed during sputtering before the target must be replaced is sometimes called the xe2x80x9ctarget utilization.xe2x80x9d Utilization is greatly affected by the maximum rate of sputtering which may be concentrated in a focused region of the target surface. Even if the average sputtering rate over the surface is small, the peak sputtering rate at a particular target location can limit the total amount of time before the target must be replaced. Thus, sputtering uniformly across the entire target surface over the target lifetime can maximize utilization.
One example of a prior art magnetron is shown in FIG. 5. The side view of FIG. 5 is a representative cross section of a target 503, including an initial front surface 507 and a back surface 505, and a horseshoe magnet 501 as used in a planar magnetron sputtering device. The magnetic poles (N and S) and magnetic field lines (dotted lines) at an initial front surface 507 are also shown. Also shown is a sputtered front surface 507xe2x80x2. During use, the target surface undergoes a loss or erosion of material due to sputtering which modifies the surface shape from initial front surface 507 to sputtered front surface 507xe2x80x2. The change in shape of front surface 507 can affect the strength of the magnetic field at the target surface, especially for a non-magnetic target material, resulting in a change in the location of further sputtering. With sputtering faster at the center than at the edge, prior art target utilization tends to be low. Utilization in many prior art magnetron sputtering devices is in the range of 17-25%
Magnets used to control sputtering and increase target utilization and lifetime are generally designed through an iterative process to select the proper size, shape, strength, and location of the magnets. Cooling requirements for the target or magnet may put further restraints on the size, material, and shape of the magnets. Obtaining an optimal design usually involves the modeling and prediction of the optimum design followed by the deposition of a number of workpieces under a variety of conditions to optimize the magnetic field. Existing magnets impose some restrictions that make design optimization difficult. For example, Bernick (U.S. Pat. No. 5,736,019, issued Apr. 7, 1998) discloses a magnet design that provides improved target utilization in some cases. While the Bernick design is an improvement over some prior art systems it incorporates tapered magnets which are expensive, difficult to cool, difficult to manufacture, and provides limited means for tuning. This complexity adds to the cost of optimizing and of manufacturing the final product. Further by way of example, Manley (U.S. Pat. No. 5,262,028) addresses the need to provide improved magnetic fields by including magnets of differing magnetic orientation, with some poles oriented parallel and other poles oriented perpendicular to the target. While this combination of magnets does allow for some modification of the magnetic field at the portion of the target being sputtered, it requires a large number of magnets, and either a further increase in the magnetic field or a decrease in the desired thickness of the target.
There is a need in the art of magnetron sputtering devices for method and an apparatus that provides higher target utilization than that associated with prior art devices. In addition, there is a need for a magnet assembly having a small number of magnets, sized and shaped for easy manufacture and assembly. There is also a need for magnets that are easily protected from the working environment and that can be cooled.
The present invention provides arrangements and methods of arranging magnets for use in magnetron sputtering devices. Typically, magnets induce magnetic field variations across the target, resulting in variations of sputtering rate. The arrangement of magnets can thus have an effect on the fraction of target material consumed during sputtering, or target utilization. Although some prior art magnets have been designed to improve target utilization, many of these include shapes, configurations or orientations that are unsuitable for effectively producing high target utilization. The arrangement of magnets of the invention and the inclusion of shunts, if needed, overcomes many of the previously described difficulties of the prior art. The use of the magnets of the invention in a magnetron sputtering device results in greater target utilization than that associated with the prior art. In particular, the present invention provides magnets that can be easily arranged to produce magnetic fields that improve target utilization. For example, while a utilization of 17-25% is common in the prior art, the present invention can provide utilization in the range of about 35-45%, with a typical value of about 40%. Increased target utilization results in a longer target lifetime, and thus has the benefit of decreased maintenance costs and operational downtime.
According to a particular aspect of the invention, magnets are arranged in groupings, or stacks, with the magnetic orientation perpendicular to the target. In one embodiment, magnets of various sizes and strengths are used to produce a closed-loop magnetic tunnel adjacent to the target. Improvements in target utilization are achieved by tuning the magnetic field according to the placement, relative to the target, of a small number of magnets. By producing a magnetic field distribution across the target that provides for a more uniform sputtering, the present invention provides a more uniform use of the target and a higher utilization. Specifically, the invention provides relatively higher fields at the edges of the target to increase sputtering rate and provides relatively lower fields in the center of the target to decrease sputtering rate. According to one aspect of the invention, magnetic fields are obtained by stacking magnets of rectangular cross-section having one side parallel to the target. According to another particular aspect of the invention, stacked magnets are provided with electrodes to produce a plasma in a magnetron sputtering device.
According to another particular aspect of the invention, magnetic fields are obtained by including shunts to modify the magnetic field at the target, either by locating the shunts between the stacks and the target to lower the magnetic field at the target, or by locating the shunts near the base or outer sides of the magnets to boost the magnetic field at the target. In one embodiment, magnets having rectangular cross-sections are stacked with one side parallel to the target, allowing for the easy placement of magnetic shunts. Specifically, shunts may be placed along stacks, across stacks, or may partially fill the space between stacks and target. This arrangement allows for the placement of shunts to tune the magnetic field, and thus improve target utilization.
According to another aspect of the invention, the stacks can be coated and/or potted to both restrain and protect the magnets from the cooling medium of the magnetron-sputtering environment or the gas within the vacuum environment. Several arrangements of magnets may be tested to optimize target utilization, and an acceptable arrangement can be potted. The potted arrangement may be incorporated into a cooling system to maintain acceptable magnet, shunt and target temperatures.
According to yet another particular aspect of the invention, a method is provided for arranging magnets in stacks to improve target utilization. The method allows for the inclusion of shunts that are used to tune the magnetic field for the purpose of optimizing target utilization and improvement of the uniformity of the sputtered film.
A further understanding of the invention can be had from the detailed discussion of specific embodiments below. For purposes of clarity, the invention is described in terms of systems that include many different innovative components and innovative combinations of components. No inference should be made to limit the invention to combinations containing all of the innovative components listed in any illustrative embodiment in this specification.
All patents cited herein are hereby incorporated by reference in their entireties for all purposes. Additional objects, advantages, aspects and features of the present invention will become apparent from the description of preferred embodiments, set forth below, which should be taken in conjunction with the accompanying drawings, a brief description of which follows.